Pressurised water nuclear reactors comprise steam generators which ensure the heating and vaporisation of feed water via the heat conveyed by the pressurised water used to cool the core of the reactor.
Pressurised water reactors, on each of their primary branches, comprise a steam generator having a primary part in which the pressurised coolant water of the reactor circulates, and a secondary part receiving the feed water which is heated and gradually vaporised and leaves the secondary part of the steam generator in the form of steam which is sent to a turbine associated with the nuclear reactor to drive an alternator producing electric current.
Said steam generators comprise an outer shell called a pressure shell of general cylindrical shape arranged with its axis positioned vertically and secured to a substantially horizontal tube sheet whose lower face or inlet face forms a wall of the plenum of the steam generator fed with pressurised water forming the primary fluid.
This plenum generally of semi-spherical shape has openings at which tubing is welded connecting with the hot branch and cold branch of a loop of the primary circuit of the nuclear reactor.
Nuclear reactors must be periodically shut down to carry out maintenance, replacement and repair operations, e.g., for refuelling operations.
This shutdown period is also used for maintenance of the steam generators.
To carry out some maintenance or repair operations on the steam generators, during shutdown periods of the nuclear reactor, it may be necessary for servicing technicians to enter inside the plenum.
The primary circuit and the vessel of the nuclear reactor are filled with water during maintenance operations on the nuclear reactor, which means that it is necessary to drain the plenum and then to shut off the primary circuit before it is possible for servicing technicians to work inside this plenum.
To enable these operators to carry out servicing inside the plenum, devices are known for the sealed plugging of the tubing of this plenum connecting with the primary circuit, these devices comprising stoppers also known as nozzle dams placed inside the tubing of the plenum prior to servicing.
To place this stopper in position, a fastening ring of the stopper is welded inside the plenum around the opening of the tubing, in the part thereof that opens onto the inside of the chamber in a coaxial arrangement relative to the tubing.
The stopper also comprises a sealing member intended to be applied onto the inner surface of the fastening ring and/or of the opening of the tubing.
It is generally necessary to form the stopper in several parts which are assembled inside the plenum, or in foldable form, since the stopper has to be inserted inside the plenum via a manhole whose diameter is generally smaller than the diameter of the opening of the tubing in which the stopper is placed.
For this purpose a stopper also known as a nozzle dam is known that is dish-shaped comprising a first cylindrical part closed by a base whose diameter is smaller than the inner diameter of the ring, and a second peripheral part of flat annular shape whose outer diameter is larger than the inner diameter of the ring.
In this case the seal is ensured by two inflatable, peripheral ring seals which are inserted between the inner surface of the ring and the outer surface of the cylindrical part of the stopper, when this stopper is in position, and by a static seal which comes to bear either against the inner surface of the opening of the tubing or against the upper surface of the ring and a peripheral retaining part in the form of a rim which comes to be applied against the outer surface of the fastening ring.
The holding in position of the stopper is generally ensured by securing members such as screws for example, which pass through the peripheral part of the stopper and are screwed into tapped orifices arranged in the ring.
The main disadvantage of this type of stopper lies in the fact that it requires the connecting of the seals to an inflating unit, and that it requires continuous monitoring of the inflation of the seals which guarantee the sealed plugging.
In addition, this type of stopper has proved to have mechanical resistance, and in particular resistance to temperature or to fluid pressure exerted on one side of the stopper, which may be insufficient for repeated use and use of long duration, or in the occurrence of accidental events.